The third ventricle (ventriculus tertius) is one of four connected fluid-filled cavities comprising the ventricular system within the human brain. It is a median cleft in the diencephalon between the two thalamus, and is filled with cerebrospinal fluid (CSF).
The posterior wall is the base of the pineal gland.
The third ventricle is a median cleft between the two thalami (see thalamus) and is bounded laterally by them and the hypothalamus. Its anterior wall is formed by the lamina terminalis, and posteriorly there is the pineal recess. There are two protrusions on the anterior surface of the third ventricle:
supra-optic recess (above the optic chiasm)
infundibular recess (above the pituitary stalk)
Anterior recesses of the third ventricle
The average length of the third ventricle ≈ 2.8 cm.